Tone control device



p 1938. J. VAN SLOOTEON 2,113,003

TONE CONTROL DEVICE Filed April '25, 1955 INVENTOR J 144/7 Jig/5W BY K ATTORNEY Patented Apr. 5, 1938 UNITED STATES TONE CONT ROL DEVICE Jacob van Slooten, Eindhoven, Netherlands, as-

signor to Radio Corpo poration of Delaware Application April 25,

ration of America, a cor- 1935, Serial No. 18,121

In Germany May 3, 1934 4 Claims.

The invention relates to circuit arrangements for acting upon the frequency characteristic curves of low frequency amplifiers employed more particularly in wireless receiving sets.

With these amplifiers it is frequently desirable to be able to give the frequency characteristic curve such a form different from the ideal faithfulness that alternating voltages supplied to the input side and whose frequencies surpass a determined limit frequency, are no longer amplified in order to suppress disturbances which may be produced, for example, in wireless reception due to interference of two neighboring transmitters. Furthermore, it is desirable to be able to vary at will the amplification for the higher tone frequencies with respect to the amplification of the lower tone frequencies.

The invention has for its object to provide a plain circuit arrangement which meets these requirements.

According to the invention, this circuit arrangement consists of a quadripole connected into the input or output circuit or between two successive amplifying tubes and having connected between its input terminals a resistance in series with a condenser and between its output terminals a condenser. Besides, an inductance is located between one of the input and one of the output terminals, the last mentioned terminal being connected to an adjustable point on the resistance located between the input terminals.

The invention will be explained more fully in connection with the accompanying drawing, in which;

Figs. 1 and 3 are schematic circuit diagrams of tone control devices embodying the invention, and

Fig. 2 shows tone control curves obtained with the arrangements of Figs. 1 and 3.

Fig. 1 shows a quadripole having pairs of terminals I, 2 and I3, I4. The low frequency alternating voltages to be amplified are supplied to the input terminals I and 2 from a source of voltage having an electromotive force E1 and an internal resistance R1. The output terminals I3, I4 are connected to the primary winding of the input transformer 5 of a low frequency amplifier of which only the first amplifying tube 6 is shown. The quadripole is composed of a series connection of a resistance R and a condenser C1, between the terminals I and 2, of an inductance L between the terminals I3 and I. Furthermore, the connecting point of the inductance L to the condenser C2 is connected by means of a conductor I to an adjustable contact 8 of the resistance R.

The manner in which this circuit arrangement functions, will hereafter be explained more fully for three different positions of the contact 8 incase the more perceptible, the higher the. frequency of the impressed alternating voltages is. This finds expression in curve I of Figure 2 which shows the ratio between the voltage E2 across the secondary winding of the transformer 5 and the voltage E1 for different frequencies. So long as the impedance 'of the condenser C2 is large relatively to the internal resistance Ri, this ratio, the voltage amplification remains nearly constant and then gradually decreases at higher frequencies at which the impedance of the condenser C2 becomes smaller and smaller. With a proper value of the condenser C2 this decrease of amplification takes place only at high frequencies and therefore it has only a slight influence on the quality of the transmission.

If the contact 8 is shifted from position I into position II, the short circuit of the inductance L is removed. In this case said inductance forms conjointly with the condenser C2 a series-resonance circuit having a determined natural frequency. For this frequency one would therefore expect a peak in the frequency characteristic curve, which however, does not become manifest in curve II of Figure 2 because that portion of the resistance R which is located between I and II causes sufficient damping. Owing to the presence of the series resonance circuit the am plification rapidly decreases for frequencies. higher than the natural frequency of the circuit, which results in that in position II disturbances having a frequency located above said limit will be suppressed.

In position III of the contact 8 the condensers C1 and C2 are connected in parallel so that the resonance circuit formed conjointly by them and the inductance L has a natural frequency which is lower than in position II. The frequency characteristic curve has in this case the form shown in curve III of Figure 2. For frequencies higher than the natural frequency of the circuit L,

C1, C2, the amplification decreases rapidly and the frequency range transmitted is considerably narrower relatively, than those in positions I and II, so that disturbances of a frequency located outside said range are suppressed. When shifting the contact 8 from position I in which both the high and the low tone frequencies are uniformly reproduced, into position III in which only the low tone frequencies are reproduced, the frequency characteristic curves shown in Figure 2 merge without interruption into one another. Thus it is possible to vary at will the timbre of the speech or music reproduced by suppressing more or less the higher tone frequencies.

In the neighborhood of the natural frequency of the circuit formed by the inductance L and the condensers C1 and C2, curve III of Figure 2 shows a peak which may be smoothed by connecting as is shown in Figure 3, a resistance R1 in series with the inductance L, which results in that in position III there may also be obtained a more regular shape of the frequency characteristic curve in the transmission range.

Having described my invention what I claim as novel and desire to secure by Letters Patent is:

1. A tone control device, comprising the combination of a source of signal currents having a fixed internal resistance and covering a band of audio frequencies, the series connection of a resistor and a condenser connected to the terminals of said source, a series resonant circuit comprising a coil and a condenser and having its opposite ends connected across the terminals of said source, said circuit having a resonant frequency near the middle of said band of frequencies and a pair of output terminals connected respectively to the opposite sides of said condenser, the impedance of said last named condenser being greater than the internal resistance of said signal source for currents having frequencies below the middle frequency of said band and an impedance less than said internal resistance for currents near the upper end of said band.

2. A circuit for transmitting a wide band of audio frequency currents comprising the combination of a transformer having primary and secondary windings, a pair of input terminals, an inductor connected between one of said terminals and one end of said primary winding, a direct connection between the other of said terminals and the other end of said primary Winding, a condenser connected across said primary winding, said condenser forming with said inductor a circuit resonant to a frequency within the band to be transmitted, a second condenser and means for connecting said second condenser in shunt with said first named condenser.

3. A circuit for transmitting a band of audio frequency currents comprising the combination of a pair of input terminals, a pair of output terminals, an inductance coil having its ends connected respectively to one input and one output terminal, a lead connecting the other input to the other output terminal, the series connection of a resistor and condenser connecting said input terminals, a second condenser connected across said output terminals, a contact adjustable along said resistor and a lead connecting said contact to the end of said coil which is con- I nected to said output terminal.

4. The circuit as defined in claim 3 in which said coil and the parallel connection of said two condensers upon adjustment of said contact to one end of said resistor form a series circuit having a resonant frequency within the band to be transmitted.

JACOB VAN SLOOTEN. 

